It is noted that basic chemical composition and/or formulae designations and the like are interchangeable with their respective generic compound, material, and/or element names throughout the following background, description, accompanying drawings, and the claims (for example: Zr is interchangeable with zirconium, O is interchangeable with oxygen, ZrO2 is interchangeable with zirconia and/or zirconium oxide, Co—Cr—Mo is interchangeable with cobalt-chromium-molybdenum, Ti is interchangeable with titanium, Y2O3 is interchangeable with yttria, and MgO is interchangeable with magnesia).
The longevity of medical implant components is of prime importance as it is desirable that an implant should function for the complete lifetime of a patient. This is particularly pertinent if the patient is young and the number of surgical revisions is to be kept to a minimum and preferably zero. To this end, orthopedic implant materials should preferably combine high strength, corrosion resistance and tissue compatibility.
One of the variables affecting the longevity of load-bearing implants such as hip implants and knee implants is the rate of wear of the articulating surfaces and long-term effects of metal ion release. For example, a typical hip-joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either the femoral head or the acetabular cup can result in an increasing level of wear particulates and “play” between the femoral head and the cup. Wear debris can contribute to adverse tissue reaction leading to bone resorption, such that ultimately the joint may need to be replaced. The rates of wear of the acetabular cup and the femoral head surfaces of artificial hips depend on a number of factors which include the relative hardness and surface finish of the materials constituting the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the loads applied, and the stresses generated at the articulating surfaces.
The most common material combinations currently used in the fabrication of hip-joint implants include femoral heads of cobalt, titanium, or zirconium alloys articulating against acetabular cups lined with organic polymers or composites of such polymers including, for instance, ultra-high molecular weight polyethylene (UHMWPE) and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or made of polished alumina. Similarly, knee, shoulder, and elbow implants typically include metallic articulating components and polymeric bearings. Of the factors that influence rates of wear of conventional load-bearing implants, most significant are patient weight and activity level. Additionally, heat generated by friction in the normal operation of an implant has been shown to cause accelerated creep and wear of bearing components. In a typical hip-joint prosthesis, there is a correlation between the frictional moment that transfers torque loading to the acetabular cup and the frictional coefficient between the femoral head and the surface of the cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. UHMWPE, being a polymeric material, is more susceptible to creep when heated than the metal alloys or ceramics commonly used to form opposing articulating surfaces due to its relatively lower melting point; consequently, it is more susceptible to wear than the alloys or ceramics.
Additionally, it has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionization process thereby releasing metal ions into the body. Metal ion release from the prosthesis is also related to the articulation and rate of wear of load bearing surfaces because, as may be expected, when a metallic femoral head, for instance, is articulated against UHMWPE, the passive oxide film which forms on the femoral head is constantly removed. The repassivation process constantly releases metal ions during this process. Furthermore, the presence of third-body wear (cement or bone debris) may accelerate this process and micro fretted metal particles can increase friction. Consequently, the UHMWPE bearing components against which some conventional metallic surfaces articulate may be subjected to additionally accelerated levels of creep, wear, and torque.
There have been attempts to layer zirconia onto articulating surfaces of metallic orthopedic implant components to provide lower friction, higher wear resistance, and/or higher corrosion resistance. Zirconia (having a basic chemical composition of ZrO2 and also sometimes called zirconium oxide) is a low-friction material that has performed positively in wear tests against polyethylene (“PE”). Another feature of ZrO2 that should be noted is its isomeric phases. The phases are identical in terms of chemistry (all involve two O atoms to one Zr atom), but differ in crystal structure: monoclinic, tetragonal and cubic. At room temperature, the stable phase of ZrO2 is the monoclinic. However, when heat is applied this transforms to tetragonal at about 950° C. Further heating of the tetragonal causes transformation to cubic at 2370° C. With additional heating the ZrO2 melts at about 2600° C. Special processing of ZrO2 can retain, at room temperature, the tetragonal or cubic phases. They are, however, thermodynamically metastable and can be transformed to monoclinic with suitable applied stress. When this happens, the ZrO2 generally spalls and fractures because of a volume change. For example, when tetragonal ZrO2 at room temperature is sufficiently stressed, its transformation to monoclinic involves a 4% volume change. Usually this causes the ZrO2 to break apart. Other influences besides stress can (such as elevated temperatures and steam exposure during autoclaving, for example) can also cause transformation. If certain species are added to the pure ZrO2, the tetragonal or even the cubic phase can be stabilized at temperatures and stress states in which they would otherwise be metastable (including room temperature). These species are called stabilizers and include yttria (Y2O3), magnesia (MgO) and calcium oxide (CaO). Addition of 2-4 wt % of any of these is sufficient to stabilize tetragonal ZrO2 at room temperature. However, it is noted that the present invention does not employ such stabilizers. Thus, ZrO2 has well established wear and friction advantages. Additionally, the inventors have observed no significant frictional or wear differences between the three phases. Nevertheless, successful uses of ZrO2 depend largely upon its compatibility with other manufacturing processes and its stability against fracture and spalling.
FIG. 1 shows a simplified illustration of an exemplary Ion Beam Assisted Deposition (“IBAD”) setup 100 and FIG. 2 shows a simplified block diagram of an exemplary IBAD process 200. IBAD, which is one process that has been used to layer zirconia onto metallic orthopedic implant components, is a generic term for employing accelerated ions to drive a vapor phase into the surface of a substrate. Other IBAD applications beyond orthopedics have included superconductor processing work (at Los Alamos National Labs, among other places), and numerous surface treatments not only for wear resistance, but also corrosion resistance, and optical modification across a broad base of industry. IBAD is disclosed in U.S. Pat. No. 5,383,934 to Armini et al., issued Jan. 24, 1995 (“Armini”), among other patents. Armini is expressly incorporated herein by reference. Implant Sciences Corp. (“ISC”) has been involved with IBAD technology as it relates to applying wear-resistance ZrO2 to orthopaedic implants.
As at least partially discernable in FIG. 1 and FIG. 2, orthopedic devices receiving IBAD treatment are typically cleaned and then loaded into a vacuum chamber. The ion gun then first bombards the device surfaces with a beam of Zr+ ions with the vapor phase off. Some of the ions implant into the part surface to create an “intermix zone” 110 (see FIG. 1) of about 1000 Å deep. In the intermix zone 110, the Zr composition changes gradually from 0% to near 100%. The intermix zone 110 contributes to good adhesion of the eventual ZrO2 layer. Next, with the ion beam still operating, neutral Zr atoms are introduced as a vapor. Simultaneously, O2 gas (or H2O vapor) is introduced. The Zr and O2 react at the substrate surface to form ZrO2, and the ion beam drives most of these molecules into the surface. After about 8 hours, a 2 μm thick surface layer of ZrO2 is formed. It is noted that this IBAD method differs from “regular” IBAD treatments in that a gas-phase reaction (Zr+O2→ZrO2) takes place. In fact, ISC prefers to call this process “ion beam reactive mixing” to distinguish it from other IBAD approaches. In any event, IBAD materials made in the ISC method have qualities and shortcomings.
FIG. 3 shows a graph 300 of exemplary X-ray diffraction patterns for transformation of tetragonal IBAD to monoclinic through application of 322 ksi peak compressive stress. One of the most negative aspects of IBAD is that the final ZrO2 is present as the metastable tetgragonal phase. This can be transformed to monoclinic via stress. Should such transformation occur in service, debris might be generated, and this might hasten wear of the PE through third-body effects.
On the other hand, the qualities of IBAD include the intermix zone (which resists delamination) and, unlike a process such as OXINIUM™, which is discussed below, IBAD is not restricted to only Zr-rich substrates. Finally, IBAD generally requires no post polishing, and may be more suitable for attachment of porous bone-in-growth materials.
Next, FIG. 4 shows a simplified block diagram of an exemplary OXINIUM™ process 400 that has been used by Smith & Nephew. OXINIUM components consist of a Zr-2.5Nb alloy with a ZrO2 surface layer of about 5 μm thickness. A typical OXINIUM component is first forged or machined from Zr-2.5Nb stock material. The component is then heated in air at about 1000° F. for about 3 hours. This is disclosed in U.S. Pat. No. 5,037,438 to Davidson, issued Aug. 6, 1991 (“Davidson”), which is expressly incorporated herein by reference. Alternatively, the component can be immersed in a molten salt bath that contains an oxidizer (typically Na2CO3) at 1290° F. for 4 hours. This is disclosed in U.S. Pat. No. 4,671,824 to Haygarth, issued Jun. 9, 1987 (“Haygarth”), which is expressly incorporated herein by reference. Whether air, salt or other, the oxidation of the Zr-2.5Nb alloy results in the ZrO2 surface layer. In addition, Davidson discloses use of other alloys, including various Zircadyne and Zircalloy grades. Here, it is also noted that the general oxidation of Zr-rich alloys for improved wear performance was disclosed at least as early as U.S. Pat. No. 2,987,352 to Watson, issued Jun. 6, 1961 (“Watson”), which is expressly incorporated herein by reference. Watson involved a bearing application that took advantage of the low-friction characteristics of ZrO2. Later, Zr and some Zr alloys became popular for use in corrosive and/or highly radioactive applications. One such alloy is Zr-2.5 Nb, known for its resistance to radiation damage. Surface oxidation of this and other Zr alloys for improved wear is disclosed in patents to Kemp (U.S. Pat. Nos. 5,316,594; 5,324,009; 5,399,207) and is a commercial offering called NOBLEIZING™. Valve parts for nuclear reactors are often made of Zr-2.5Nb (or similar) and subjected to a NOBLEIZING treatment.
OXINIUM also has both qualities and shortcomings as a bio-material. A noteworthy quality is that the ZrO2 that forms during oxidation is in the stable monoclinic phase. OXINIUM is therefore resistant to stress/thermally-induced transformation in regular usage. This, in turn, removes risks of transformation-induced fracture and spalling due to normal orthopaedic stress and/or thermal situations. On the other hand, OXINIUM has notable shortcomings, including a ZrO2 delamination potential, subsequent exposure of an inferior alloy (Zr-2.5Nb), a reported need to polish components after oxidation, and difficulty in attaching porous bone in-growth materials.
Thus, there is a general need for metallic orthopedic implant components with low friction, wear resistant, and corrosion resistant articulating surfaces and, more particularly, there is a need for a method for producing zirconia-layered orthopedic implant components that provides qualities similar to those of IBAD and OXINIUM without some of their shortcomings.